Encephalopathy, with psychiatric and neurological signs and symptoms, is common in HIV infections in humans. A Simian Immunosupressive Virus (SIV) in rhesus monkeys also produces encephalopathy as well as immunosuppression. SIV in monkeys could therefore provide a valuable system for mechanistic and therapeutic studies of the CNS effects of a lentivirus, closely similar to HIV, in a primate related to man in which sophisticated behavioral assessments can be made. Monkeys will be trained in a number of tasks homologous to the functions that are impaired in HIV infections of the CNS in humans. When a steady state baseline has been obtained, the monkeys will be inoculated with SIV and the behavioral assessments continued until death of the subject. Detailed neuropathology will then be conducted. When the natural history of the progression has been adequately characterized subjects will be killed at different stages of the disease so that the progressive behavioral deficits can be related to the progressive neuropathological changes early in the disease and without the complication of terminal infections. The behavioral assessments will include: responding under FI schedules; a Delayed matching-to-sample procedure; a Strength test requiring sustained strong isometric work; a Steadiness test requiring a subject to hold a rod away from the edges of a hole; and a Visual scanning task that requires a subject to recognize a "g" or and "I" in large matrices of letters that include "q"s and "l"s.